Peroxidase-catalyzed formation of quercetin quinone methide-glutathione adducts

The oxidation of quercetin by horseradish peroxidase/H2O2 was studied in th e absence but especially also in the presence of glutathione (GSH), HPLC an alysis of the reaction products formed in the absence of GSH revealed forma tion of at least 20 different products, a result in line with other studies reporting the peroxidase-mediated oxidation of flavonoids. in the presence of GSH, however, these products were no longer observed and formation of t wo major new products was detected. H-1 NMR identified these two products a s 6-glutathionylquercetin and 8-glutathionylquercetin, representing glutath ione adducts originating from glutathione conjugation at the A ring instead of at the B ring of quercetin, Glutathione addition at positions 6 and 8 o f the A ring can best be explained by taking into consideration a further o xidation of the quercetin semiquinone, initially formed by the HRP mediated one-electron oxidation, to give the o-quinone, followed by the isomerizati on of the o-quinone to its p-quinone methide isomer. All together, the resu lts of the present study provide evidence for a reaction chemistry of querc etin semiquinones with horseradish peroxidase/H2O2 and GSH ultimately leadi ng to adduct formation instead of to preferential GSH-mediated chemical red uction to regenerate the parent flavonoid. (C) 2000 Academic Press.